The present invention relates in general to integrated circuits, and in particular to a versatile and efficient transmission line driver circuit with high output impedance at power off.
Integrated circuits developed for applications such as data or telecommunication systems are often required to comply with certain transmission protocols and standardized interface specifications. The interface requirements vary depending on the standard. For example, the V.35 transmission line interface standard specifies a differential output signal of .+-.0.55 volts driving a T-shaped network of 50 Ohm resistors, while the V.28 (or RS232) standard requires a single-ended output voltage having an absolute magnitude larger than 5 volts, and exhibiting an output impedance greater than 300 ohms. These varying requirements, therefore, dictate the use of two separate driver circuits in order to comply with the separate standards.
Referring to FIG. 1, there is shown a typical current-output V.35 transmission line driver circuit. The circuit includes two current sources I1 and I2 each drawing a current Io, a network of current-steering transistors M1, M2, M3, and M4, and termination resistors R1, R2, and R3. Resistors R4, R5, and R6, constitute a transmission line termination at the far end of the line 100. Ideally, the resistances R1=R2=R4=R5=R=50 Ohms, and R3=R6=125 Ohms. In operation, when control signal C is high, transistors M2 and M3 turn on. As a result, the voltage at terminal T2 will be more positive than that of T1 by a voltage difference of 2R.times.Io. Conversely, when C is low, the voltage at terminal T1 will be more positive than that of T2 by a voltage difference if 2R.times.Io. This assumes that due to the matching of both resistive terminations, current Io splits equally between them.
A drawback of this circuit is that under power off conditions when both positive and negative power supply voltages are at ground, i.e., VDD=VSS=0 volt, it may cause excessive current drawn from transmission line 100. If the voltage levels at terminals T1 and T2 rise or fall one diode drop above or below ground under such a power off condition, intrinsic pn junctions in the filed effect transistors of the driver circuit would be forward biased drawing excessive currents from the terminals.
In order to provide a circuit that complies with different transmission line protocols, some circuits combine the various drivers on the same chip. For a transmitter-receiver circuit, for example, it is desirable to be able to share resources to achieve programmability, or to otherwise reduce cost (reduce package pin count, etc.). However, the power off current associated with the V.35 driver circuit shown in FIG. 1 would make this circuit unacceptable when the same output terminals are to be shared by, for example, an RS232 (V.28) driver. Therefore, this circuit would not allow the transmitter-receiver chip to programmably connect the appropriate driver the same pair of output pins that connect to the transmission line.
There is a need for circuit techniques that allow the sharing of the same output terminals between line driver circuitry complying with different transmission protocols.